Buy windshield (windscreen) washer fluid. You have to open the hood (bonnet) of the car, look for the cap that says "washer fluid" (it usually has the same symbol as the button/lever in your car) and fill that container up.
Washer bottle on a C5 sits under the driver's side headlamp and needs front bumper removal to access it. This is alot easier than it sounds and only took me 10 mins to do. I recently had to replace...